Unloading system for hydraulic circuit

ABSTRACT

An unloading system including an unload valve draining a part of supplied pressurized fluid to a tank and a switching valve for draining a pressurized fluid for biasing the unload valve in a direction for reducing a flow rate of the fluid supplied to the tank. The unload valve and the switching valve are arranged in series in axial direction in a valve main body.

FIELD OF THE INVENTION

The present invention relates to an unloading system to be employed in ahydraulic circuit for supplying a discharged pressurized fluid of ahydraulic pump to an actuator by a direction control valve.

BACKGROUND ART

In a hydraulic circuit employing a closed center type direction controlvalve which is designed for closing an inlet port at a neutral position,flow of a discharged pressurized fluid of the hydraulic pump is shut offin the direction control valve to elevate the discharge pressure. As aresult, power loss of a driving power source for driving the hydraulicpump becomes large.

Therefore, as a system for reducing the power loss, an unloading systemshown in Japanese Examined Patent Publication (Kokoku) No. Heisei2-53641 has been known, for example.

Namely, it has been known that an unloading system which comprises anunload valve including a pressure receiving portion and a valve portionvarying an opening area depending upon variation of the pressure at thepressure receiving portion, and a switching valve, is provided. Theswitching valve is opened while the direction control valve is held inthe neutral position, for establishing fluid communication between adischarge passage of the hydraulic pump and a tank. The switching valveis closed upon switching of the valve position of the direction controlvalve from the neutral position to the supply position. In conjunctiontherewith, the opening area of the valve portion is varied by a loadpressure acting on the pressure receiving portion of the unload valve tobranch a part of the discharged pressurized fluid of the hydraulic pumpto the tank to make a pressure difference at the upstream and downstreamof the direction control valve constant.

Such unload system encounters a problem in complicated constructionsince the unload valve constituted of the pressure receiving portion andthe valve portion and the switching valve are separated.

The present invention is to overcome such defect in the prior art.Therefore, it is an object of the present invention to provide an unloadsystem for a hydraulic circuit which can significantly simplify theconstruction.

DISCLOSURE OF THE INVENTION

In order to accomplish the above-mentioned object, according to oneaspect of the invention, there is provided an unloading system which ischaracterized in that an unload valve draining a part of suppliedpressurized fluid to a tank and a switching valve for draining apressurized fluid for biasing the unload valve in a direction forreducing a flow rate for draining to the tank, are arranged in series inan axial direction in a valve main body.

With the construction set forth above, since the unload valve and theswitching valve are arranged in axial direction of the valve main body,the construction can be significantly simplified.

It should be noted that, in the construction set forth above, it isdesirable that the unload valve is constructed by forming a valve borehaving an inlet port, a tank port and an outlet port in the main valvebody, fitting a valve body for communicating the inlet port, the tankport and the outlet port, and for increasing and decreasing open areabetween the inlet port and the tank port, in one side of the valve bore,and forming a first pressure receiving chamber communicated with theinlet port, in which a pressure biases the valve body in a direction forreducing the open area, in the valve bore, and the switching valve isconstructed by engaging a spool at the other side of the valve bore fordefining a second pressure receiving chamber to be supplied a loadpressure between the spool and the valve body, and providing a springfor biasing the spool in a direction for communicating the firstpressure receiving chamber to the tank against the load pressure of thesecond pressure receiving chamber.

Furthermore, it is preferred that the first pressure receiving chamberis defined between the valve body, a valve main body and the plug andthe second pressure receiving chamber is defined between the valve body,the plug and the spool, by engaging and fixing a plug to the other sideof the valve bore, engaging a small diameter portion of the valve bodyat one side of an axial bore of the plug, and engaging the spool to theother side of the axial bore, the first pressure receiving chamber iscommunicated with the inlet port via a cut-out of the valve body, afirst fluid bore communicated with the tank and a second fluid borecommunicated with the first pressure receiving chamber are formed in theplug, and a communication between the first and second fluid bores isestablished and blocked by the spool.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given herebelow and from the accompanying drawings of thepreferred embodiment of the invention, which, however, should not betaken to be limitative to the present invention, but are for explanationand understanding only.

In the drawings:

FIG. 1 is an illustration showing a hydraulic circuit having oneembodiment of an unloading system according to the present invention;and

FIG. 2 is a section of the above-mentioned embodiment.

BEST MODE FOR IMPLEMENTING THE INVENTION

The preferred embodiment of an unloading system according to the presentinvention will be discussed hereinafter with reference to theaccompanying drawings.

FIG. 1 is an illustration showing a hydraulic circuit having oneembodiment of an unloading system according to the present invention. Inthe illustrated hydraulic circuit, a discharge passage 2 of a hydraulicpump 1 is connected to an inlet port 4 of a direction control valve 3via a check valve 5. First and second actuator ports 6 and 7 of thedirection control valve 3 are connected to an actuator 10 via first andsecond circuits 8 and 9, respectively. An unload valve 11 is provided inthe discharge passage 2.

A valve body of the unload valve 11 is biased in a direction to increasean open area by a pressure to be applied to a first pressure receivingportion 12 and in a direction to reduce the open area by pressuresrespectively applied to second and third pressure receiving portions 13and 14.

An orifice 15 is provided at a position at the side of the hydraulicpump 1 relative to the unload valve 11 in the discharge passage 2. Thepressure at the upstream side of the orifice 15 is introduced into thesecond pressure receiving portion 13 of the unload valve 11 via acircuit 16. On the other hand, the pressure at the downstream side ofthe orifice 15 is introduced into the first pressure receiving portion12. Thus, the open area of the unload valve 11 may be increased ordecreased depending upon the pressure difference between upstream anddownstream of the orifice 15. Also, the second pressure receivingportion 13 of the unload valve 11 is connected to a tank 23 via acircuit 18 including a switching valve 17. A valve body of the switchingvalve 17 is maintained at a communicating position C by means of aspring 19, and is adapted to be switched into a shut-off position D by aload pressure P₂ exerted on a pressure receiving portion 20.

The third pressure receiving portion 14 and the pressure receivingportion 20 are connected to a load detection circuit 21.

Next, the operation of the foregoing embodiment will be discussed.

When the valve body of the direction control valve 3 is placed at apressurized fluid supply position B, the load pressure P₂ is generatedin the load pressure detecting circuit 21 to place the valve body of theswitching valve 17 at the shut-off position D. Thus, a dischargepressure P₁ is supplied to the second pressure receiving portion 13 ofthe unload valve 11. Then, when a revolution speed of the engine 22becomes higher to increase a discharge amount of the hydraulic pump 1per unit period, the pressure difference between upstream side anddownstream side of the orifice 15 is increased. Thus, the valve body ofthe unload valve 11 is biased in the direction for decreasing the openarea so as to reduce the fluid amount to recirculate to the tank 23 toincrease the supply fluid amount to the actuator 10. Also, the pressuredifference between the discharge pressure P₁ and the load pressure P₂ isincreased.

On the other hand, the revolution speed of the engine 22 is lowered todecrease the discharge amount per unit period and the pressuredifference between upstream and downstream sides of the orifice 15 isreduced. Therefore, the valve body of the unload valve 11 is biased inthe direction for increasing the open area to increase the fluid amountrecirculated to the tank 23 and thus to reduce the fluid amount to besupplied to the actuator 10. Thus the pressure difference between thedischarge pressure P₁ and the load pressure P₂ becomes smaller.

Thus, a supply fluid amount to the actuator depending upon the enginerevolution speed can be obtained.

On the other hand, by varying the resistance value of flow restrictionof the orifice 15 or the pressure receiving area of the pressurereceiving portion of the unload valve 11, the rated value of a setpressure difference of the unload valve can be varied.

On the other hand, when the valve body of the direction control valve 3is operated from the pressurized fluid supply position B to the neutralposition A, the pressure of the load pressure detecting circuit 21becomes zero to place the valve body of the switching valve 17 at adrain position C. Therefore, the discharge pressure P₁ to be supplied tothe second pressure receiving portion 13 of the unload valve 11 flows tothe tank 23. As a result, the valve body of the unload valve 11 isbiased in the opening direction to be fully opened. Therefore, thedischarge pressure P₁ becomes the pressure difference to be caused inthe fixed orifice in the unload valve 11. Thus, the power loss of theengine 22 at the neutral position A of the direction control valve 3 canbe reduced.

It should be noted that the orifice 25 is provided in the circuit 16 sothat pressure is generated in the discharge passage 2 when the circuit16 is connected to the tank 23.

Next, the concrete construction of the unloading system constituted ofthe unload valve 11 and the switching valve 17 set forth above will bediscussed with reference to FIG. 2.

In the shown unloading system, at one side of a valve bore 31 of a valvemain body 30, a valve body 34 for establishing and blocking acommunication between an inlet port 32, a tank port 33 and an outletport 53 is engaged. On the valve body 34, an axial bore 35, a first port36 and a second port 37 are formed. The inlet port 32 and the axial bore35 are in communication via the first port 36. In conjunction therewith,the opening area between the tank port 33 and the second port 37 isincreased and decreased depending upon the sliding position of the valvebody 34. These form the unload valve 11.

At the other side of the bore 31, a plug 38 is inserted and fixed. Asmall diameter portion 40 of the valve body 34 of the unload valve 11 isengaged in one side of an axial bore 39 of the plug 38. Also, at theother side of the axial bore 39, a spool 41 forming the switching valve17 is engaged so as to define a pressure receiving chamber 42 betweenthe small diameter portion 40 and the spool 41. The pressure receivingchamber 42 communicates with a load pressure detecting portion 44 via aconduit 43. The pressure receiving chamber 42 is the pressure receivingportion 20 communicated with the third pressure receiving portion 14.

In the spool 41, a small diameter portion 47 a small diameter portion isprovided for establishing and blocking communication between a firstfluid bore 45 and a second fluid bore 46. The spool 41 is biased in adirection for establishing communication by means of a spring 48 (spring19 in FIG. 1) and in a direction for blocking communication by thepressure in the pressure receiving chamber 42.

The first fluid bore 45 communicates with the tank 23. The second fluidbore 46 communicates with a pressure receiving chamber 49 definedbetween the valve body 34 and the plug 38. The pressure receivingchamber 49 communicates with the inlet port 32 via a cut-out 50 of thevalve body 34. The cut-out 50 and the second fluid bore 46 are thecircuits 16 and 18 in FIG. 1, respectively. Also, the chamber 49 is thefirst pressure receiving portion 13, in FIG. 1. Also, the cut-out 50 isthe orifice 25 in FIG. 1. On the other hand, the axial bore 35 is thesecond pressure receiving portion 12. Reference numeral 53 denotes anoutlet port connected to the inlet port 4 of the direction control valve3.

As set forth above, the unload valve 11 and the switching valve 17 arearranged in series in the axial direction of the valve main body 30.

It should be noted that, in FIG. 2, reference numeral 51 denotes arelief valve connected to the load detection port 44 and 52 denotes asafety valve.

Next, the operation of the disclosed embodiment will be discussed.

At first, when the direction control valve 3 is held in the neutralposition A, the load pressure is not supplied to the load detecting port44. Thus, the spool 41 is biased toward the left as viewed in FIG. 2 bythe spring 48 to establish communication between the second fluid bore46 and the first fluid bore 45 via the small diameter portion 47.Therefore, the pressure of the pressure receiving chamber 49 is loweredto reduce the force required to bias the valve body 34 toward left sideas shown in the figure.

The valve body 34 slides toward right (in unload direction) due to thepressure in the axial bore 35 to increase an open area between thesecond port 37 and the tank port 33 so as to lower the dischargepressure of the hydraulic pump 1 to a pressure level greater than thepressure of the tank port 33 in the extent corresponding to the pressuredifference caused in the first port 36 of the valve body 34 (the orifice15 in FIG. 1).

Next, when the direction control valve 3 is switched from the neutralposition A to the supply position B, the load pressure is supplied tothe load pressure detecting port 44, the spool 41 is biased toward rightby the load pressure in the pressure receiving chamber 42 against thespring 48 to block the communication between the first fluid bore 45 andthe second fluid bore 46. Thus, by the discharge pressure supplied tothe pressure receiving chamber 49, the force biasing the valve body 34toward the left (in on-load direction) becomes greater.

By this, the valve body 34 slides toward left (in on-load direction) toreduce the open area between the second port 37 and the tank port 33.Thus, the discharge pressure of the hydraulic pump 1 is elevated.

The unload system according to the present invention, operation of whichhas been discussed above, can simplify the construction thereof sincethe unload valve 11 and the switching valve 17 are arranged in series inthe valve main body 30.

Although the invention has been illustrated and described with respectto exemplary embodiment thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions and additions may be made therein and thereto, withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention should not be understood as limited to thespecific embodiment set forth above but to include all possibleembodiments which fall within a scope encompassed and equivalentsthereof with respect to the feature set out in the appended claims.

What is claimed is:
 1. An unloading system comprising:a valve main body;an unload valve, for draining a part of supplied pressurized fluid to atank, said unload valve being defined by a valve bore formed in saidvalve main body and a valve body, said valve bore having an inlet port,a tank port and an outlet port, said valve body being fitted in one sideof said valve bore for communicating with the inlet port, the tank portand the outlet port, and for increasing and decreasing an open areabetween said inlet port and said tank port, and for forming a firstpressure receiving chamber in communication with said inlet port, suchthat a pressure biases said valve body in a direction of said valve borefor reducing the open area; and a switching valve, arranged in serieswith said unloading valve in an axial direction of said valve main body,for draining a pressurized fluid for biasing said unload valve in adirection so as to reduce a flow rate to the tank, said switching valvebeing defined by a spool inserted in the other side of said valve borefor defining a second pressure receiving chamber to be supplied with aload pressure between said spool and said valve body, and a spring forbiasing said spool in a direction so as to establish communicationbetween said first pressure receiving chamber and said tank against theload pressure of said second pressure receiving chamber.
 2. An unloadingsystem as set forth in claim 1, wherein said first pressure receivingchamber is defined between said valve body, said valve main body and aplug engaged and fixed to the other side of said bore, and said secondpressure receiving chamber is defined between said valve body, said plugand said spool, a small diameter portion of said valve body is engagedat one side of an axial bore of said plug, and said spool slidablyengaged in the other side of the axial bore of said plug, said firstpressure receiving chamber communicates with said inlet port via acut-out of said valve body, a first fluid bore is formed in said plug soas to provide communication with said tank and a second fluid bore isformed in said plug so as to provide communication with said firstpressure receiving chamber, and communication between said first andsecond fluid bores is established and blocked by said spool.